1. Field of the Invention
The present invention relates to a PRAM and a method of firing memory cells in same.
2. Description of the Related Art
A Phase change Random Access Memory (PRAM) stores data using a phase change material, such as a chalcogenide alloy. The logic state of the stored data is determined by the physical state of the phase change material (e.g., a crystalline state verses an amorphous state). Contemporary phase change materials alter their physical state under defined thermal conditions (e.g., applied heating and cooling conditions). One common form of phase change material exhibits low electrical resistance in a crystalline state and a high resistance in an amorphous state. The crystalline state has been associated with a “set” logic value (i.e., a logic value of 0). The amorphous state has been associated with a “reset” value (i.e., a logic value of 1).
The fabrication of a PRAM, like all other semiconductor memory devices involves the complex sequence of fabrication processes. However, unique to the fabrication of PRAM devices, is the process of “firing”. The firing process applies one or more electrical pulses of relatively significantly magnitude to a phase change memory cell. More specifically, following the fabrication of the phase change memory cell, the constituent phase change material resides in a mixed amorphous state, including material in a Face-Centered-Cubic (FCC) state, and other material in an Hexagonally-Close-Packed (HCP) state. A principal object of the firing process is to place all of the phase change material uniformly in the FCC state. Once the phase change material is placed in the FCC state, transition characteristics associated with the material may be readily predicted (i.e., conditions associated with changing the material between an amorphous state and a crystalline state may be predicted with good accuracy). This predictability allows for improved sensing margins for the constituent phase change memory cell.
The foregoing assumes that that the “firing pulse” applied during the firing process are of a sufficient magnitude to uniformly change the initial state of the phase change material. For this assumption to hold, a firing pulse of significant magnitude must be applied. However, application of a large magnitude firing pulse runs the risk of damaging one or more of the weaker phase change memory cells in a memory cell array.
Additionally, conventional PRAMs include a circuit block specifically designed to facilitate the firing process. This requirement expands the size of conventional PRAMs in direct opposition to considerable ongoing efforts to minimize its size.